Advanced Configuration and Power Interface (ACPI) is a body of power management software, typically integrated with or integrated to work closely with, an operating system (OS) of a computer. ACPI provides for the definition of “performance states”, e.g., P0 through P5, that define performance levels of an operating (as opposed to sleeping) computer system. Each performance state typically defines a specific operating frequency of the computing system's underlying processor. As is known in the art, the performance level (and power consumption) of a processor correlates with its operating frequency. Essentially, the faster a processor operates, the more work it can do in a specific time period (higher performance level) and the more power it consumes.
Typically, a system manufacturer integrates a computer's ACPI software and processor together and defines the specific performance states for the processor through the BIOS firmware of the system. That is, definitions of the specific ACPI performance states together with the corresponding processor frequency of each performance state are stored in BIOS firmware. During boot-up of the system, the performance state information is read from BIOS firmware. During operation of the computer, the ACPI power management software uses the information to cause appropriate performance state transitions by issuing commands that change the processor's frequency.
Typically, the ACPI power management software monitors the workload of the processor. If the workload becomes extremely heavy, the ACPI power management software will issue a command that causes the processor to switch to its highest supported operating frequency/performance state (e.g., P0). If the workload becomes very light but not non-existent, the ACPI power management software will issue a command that causes the processor to switch to the lowest frequency at which it can still execute application software (e.g., P5).